COVALENT BONDS AND MOLECULAR STRUCTURES Atoms are held together by electrostatic interactions after complete transfer of one or more electrons Ionic bonds are formed Remember the slides showing the formation of ionic solids i e NaCl Atoms of the same type will not form an ionic solid by transferring an electron Instead electrons are equally shared between the atoms in what we call a covalent bond Atoms held together by covalent bonds make up molecules Why does hydrogen form H2 molecules if there is no electrostatic energetically VERY favorable interaction to bond the atoms We must look at ALL interactions when two H atoms approach each other The two nuclei repel each other The two electrons repel each other But each nucleus attracts has a favorable interaction with BOTH electrons Depending on the exact distance attractive forces can be stronger than repulsive forces The nucleus electron attractions are greater than the nucleus nucleus and electron electron repulsions resulting in net attractive forces If atoms are too close strong repulsion occurs If atoms are too far apart attraction are too weak to form a bond If atoms are optimally separated energy is at a minimum Formation of covalent bonds lowers the energy of the resulting molecule compared to the sum of energy levels for the constituent atoms i e 436 kj mol of energy is released when H2 is formed LOOK AT FIG 7 2 Breaking a covalent bond requires the same amount of energy called bond dissociation energy Bond dissociation energies are different depending on the molecules but bonds between the same two elements are usually similar in energy COMPARING IONIC AND COVALENT COMPOUNDS Ionic solids are crystals held together by lattice energy All interactions must be overcome to melt or boil the ionic solid Covalent solids are molecules held together by covalent bonds Interactions between molecules are weak and easy to overcome The strong covalent bonds remain intact during melting boiling Example of ionic bond is NaCl Example of covalent bond is HCl Ionic bonds are stronger than covalent bonds CLOSER LOOK AT DIFFERENT BONDS Two extremes have been discussed so far Complete transfer of electrons from one atom to another resulting in an ionic solid Equal sharing of electrons between two atoms resulting in a covalent bond A large majority of bonds are POLAR Electrons are unequally shared Denoted by the greek letter delta An ability of an atom in a molecule to attract electrons is given by its electronegativity Electronegativity increases from left to right and bottom to top F has highest electronegativity with 4 0 VISUALIZING POLAR AND NON POLAR BONDS Electrostatic potential maps help visualize electron distribution in molecules Green Yellow Non polar neutral Red Orange Partial negative charge Blue Purple Partial positive charge Also uses size to indicate probability of finding electrons around an atom ELECTRONEGATIVITY AND TYPE OF BONDING The difference in electronegativity between two bonded pairs can be used to estimate whether the bond will be ionic polar covalent or non polar covalent If difference in electronegativity is greater than 2 0 the bond is mainly ionic For ions we have indicated the location of electrons by writing ionic charges Together with the ground state electron configuration we know exactly how many electrons are around which atom In covalent compounds we only worry about the valence electrons Just like in ionic compounds core electrons are unlikely to participate in bonding To indicate a covalent bond we either draw a line connecting 2 atoms or so called electron dot structures Lewis structures show covalent and ionic bonds Dots represent lone pairs and bonding pairs OCTET RULE AND ELECTRON DOT STRUCTURES Many molecular structures can be predicted by keeping in mind that main group elements strive to achieve an octet of valence electrons A main group element will share its valence electrons until it reaches octet configuration Most elements reach an octet Group 3A elements are an exception because they only have 3 electrons to share